The present invention relates generally to electronic engine controls and to feedback controls for engine operation using exhaust gas oxygen sensors. More particularly, the present invention relates to a sensor system having an exhaust gas oxygen ("EGO") sensor interconnected to an exhaust system downstream of a catalytic converter.
Many automotive vehicles include an internal combustion engine and an exhaust system that provides a conduit for heated combustion gas to move away from the engine. The temperature of the exhaust gas ranges from ambient temperature, when the engine has not been in operation recently, to 400.degree. Celsius or more.
A typical exhaust system may include an EGO sensor assembly and a catalytic converter. The catalytic converter promotes the conversion of hydrocarbons, carbon monoxide, and oxides of nitrogen into less noxious compounds. An EGO sensor is often placed "upstream" of the catalytic converter. The terms "downstream" and "upstream" are relative terms used to denote relative positions along the exhaust conduit, or pipe, of the vehicle. The term "downstream" refers to positions along the exhaust conduit that are reached by a particle in the exhaust gas later in time than positions that are "upstream."
Many air-fuel control systems in presently available vehicles, with the EGO sensor located upstream of the catalyst, provide an air-fuel feedback signal for a closed-loop air-fuel delivery system in the engine. The upstream EGO sensor, however, can be "poisoned" by certain compounds, such as lead or silicone. Such components may be present in the raw exhaust gas. This may occur, for example, if a motorist improperly uses "leaded" gasoline in an engine designed only for "unleaded" gasoline. Such poisoning may render the EGO sensor ineffective in accurately ascertaining the level of the oxygen concentration in the exhaust gas.
Also, the output characteristics of an upstream EGO sensor may change over time. Moreover, under some operating conditions, the upstream EGO sensor may be unable to bring the exhaust gas flowing nearby it to a substantial equilibrium. Such conditions may be dependent on, for example, the engine load and cylinder-to-cylinder air-fuel maldistribution in the engine. As a result, the EGO sensor will exhibit "offset errors."
Further, many EGO sensors only operate effectively if the temperature of the sensor is within a particular range. The temperature of the sensor is, of course, influenced by the temperature of the adjacent exhaust gas. To assist an EGO sensor to make accurate measurements over a wide range of exhaust gas temperatures, the EGO sensor assembly often includes an electric heater physically adjacent, or near, the EGO sensor. Such a heated exhaust gas oxygen sensor is a type of EGO sensor and is often referred to as a HEGO sensor. When actuated, the heater warms the sensor to enable it make more accurate measurements and, thus, reduce the effect of temperature variations of the exhaust gas passing through the exhaust pipe of the vehicle.
Prior art systems exist for controlling the air-fuel ratio of an internal combustion engine. For example, U.S. Pat. No. 4,708,777, issued to Kuraoka, discloses an air/fuel ratio feedback control system that is responsive to an EGO sensor. The EGO sensor is maintained at a predetermined temperature by feedback from the sensor heater.
Thus, some prior systems have attempted to maintain a constant air-fuel ratio operating point, which is independent of the exhaust gas temperature. In addition to maintaining a constant, closed-loop air-fuel ratio operating point independent of exhaust gas temperature or engine operating conditions, however, it is also desirable to have an EGO sensor that may more accurately detect oxygen levels, regardless of the exhaust gas constituencies and poisoning effects. In this way, the feedback control enables the controller to more precisely regulate the operation of the internal combustion engine.
Further, since the EGO sensor assemblies are generally mass-produced and put on many cars, even a small savings on one part of the assembly can accumulate to a substantial annual savings. Thus, an EGO sensor system should not have an excessive number of parts nor high manufacturing costs. Moreover, it is important that the sensor assembly be reliable.